1. Field of the Invention
The invention relates in general to a frequency synthesizer and a frequency synthesizing method, and more particularly to a frequency synthesizer and a frequency synthesizing method for converting spurious tones into noise.
2. Description of the Related Art
A frequency synthesizer provides a precise and stable frequency source within a frequency range, which is frequently seen in a modern electronic device, such as a radio receiver, a mobile telephone, a satellite receiver, a global positioning system or the like. The frequency synthesis technique includes the direct frequency synthesis, the indirect synthesis and the like. The direct frequency synthesis technique includes the direct digital pulse rate architecture, the flying-adder architecture, and the like, and has the advantages of the high frequency changing speed and the capability of generating the arbitrary frequency. However, it tends to generate spurs, and thus causes a frequency spectrum of an output signal of an analog-to-digital converter or a digital-to-analog converter, controlled by the frequency synthesizer, to contain a non-essential spurious tone.
The generation of the spur is substantially caused by a regular carry sequence of an accumulator in the frequency synthesizer.
Please refer to FIGS. 1A to 1C. FIG. 1A (Prior Art) is a schematic illustration showing an accumulator 05 of a conventional frequency synthesizer. FIG. 1B (Prior Art) a waveform graph showing an accumulated result of the accumulator of the conventional frequency synthesizer corresponding to FIG. 1A. FIG. 1C (Prior Art) shows signal frequency spectrums of carry sequences corresponding to the conventional frequency synthesizer of FIG. 1A and different fractional part values. In FIG. 1A, the conventional accumulator 05 adopts a control character FREQ, including an integer value I and a fractional part value r, to generate a carry sequence, wherein the fractional part value r is a constant. Because the fractional part value r is the constant, the accumulated result is stably increased and the carry sequence, generated by the conventional accumulator 05, has the regularity, as shown in FIG. 1B. After observing FIG. 1C, it is obtained that the spur, caused by the regular carry sequence, makes the frequency spectrum of the carry sequence contain non-essential spurious tones. In addition, the spurious tones caused by different fractional part value r are also different.
Please refer to FIGS. 1D and 1E. FIG. 1D (Prior Art) is a schematic illustration showing another example of an accumulator 10 of the conventional frequency synthesizer. FIG. 1E (Prior Art) is a waveform graph showing an accumulated result of the accumulator of the conventional frequency synthesizer corresponding to FIG. 1D. In FIG. 1D, the conventional accumulator 10 adopts the method of adding a random number v to the control character FREQ, including the integer value I and the fractional part value r, in order to break the regularity of the carry sequence, as shown in FIG. 1E, and thus to improve the generation of the spur and achieve the result of random dithering.
However, the method of adding the random number to the control character has many drawbacks. For example, the optimum size of the random number and the optimum adding rate cannot be easily determined, but the try and error method has to be adopted to repeat the operation to obtain the optimum value. In addition, in order to keep the output frequency unchanged, the overall average of the random numbers must be equal to 0. Thus, the circuit design has to use the signed number system, and the system complexity and cost are increased. Furthermore, in the high-speed operation mode, a high-speed adder needs to be used to add the random number, thereby wasting a lot of hardware resources.